Vibrating flow devices such as, for example, densitometers and Coriolis flow meters are used for measuring a characteristic of flowing substances, such as, for example, density, mass flow rate, volume flow rate, totalized mass flow, temperature, and other information. Vibrating flow devices include one or more conduits, which may have a variety of shapes, such as, for example, straight, U-shaped, or irregular configurations.
The one or more conduits have a set of natural vibration modes, including, for example, simple bending, torsional, radial, and coupled modes. The one or more conduits are vibrated by at least one drive at a resonance frequency (hereinafter referred to as the “drive frequency”) in one of these modes, (hereinafter referred to as the “drive mode”), for purposes of determining a characteristic of the flowing substance. One or more electronics transmit a sinusoidal drive signal to the at least one drive, which is typically a magnet/coil combination with the magnet typically being affixed to the conduit and the coil being affixed to a supporting structure or to another conduit. The drive signal causes the drive to vibrate the one or more conduits at the drive frequency in the drive mode. For example, the drive signal may be a periodic electrical current transmitted to the coil.
At least one pick-off detects the motion of a conduit and generates a sinusoidal pick-off signal representative of the motion of the vibrating conduit(s). The pick-off signal is transmitted to the one or more electronics; and according to well known principals the pick-off signal may be used by the one or more electronics to determine a characteristic of the flowing substance or adjust the drive signal, if necessary.
Vibrating flow devices may also include a housing that typically encompasses the driver(s), the pick-off(s), and the conduit(s). Housings are typically used for a variety of reasons, including, for example, to provide a stable, known, or controlled operating environment, i.e. an environment free of moisture and harmful gases, or to protect the conduit(s) driver(s) or pick-off(s), i.e. from moisture, debris, or from damage due to contact with other objects or during shipment.
Housings also have one or more natural modes of vibration, including, for example, simple bending, torsional, radial, and lateral modes. In FIG. 4, for example, the bending mode would be about axis B and a lateral mode would be about axis A. As shown in FIG. 5, the particular frequency which induces a mode of vibration will vary. The frequency may vary according to a variety of factors including, for example, fluid density, as FIG. 5 shows, or environmental conditions, such as temperature. Vibrational forces generated by the driver and from other sources in the material processing system, such as pumps, may cause the housing to vibrate in one of the natural modes. Where the frequency used to drive the one or more conduits in the drive mode corresponds to the frequency that causes the housing to vibrate in one of its natural housing modes of vibration it becomes difficult to generate an accurate measurement of a characteristic of the flowing substance.
The present invention is directed to overcoming this disadvantage inherent in prior art housings.